Elisabetta Di Bartolomeo

High proton conduction in grain-boundary-free yttrium-doped barium zirconate films grown by pulsed laser deposition

Reducing the operating temperature in the 500-750 °C range is needed for widespread use of solid oxide fuel cells (SOFCs). Proton-conducting oxides are gaining wide interest as electrolyte materials for this aim. We report the fabrication of BaZr(0.8)Y(0.2)O(3-δ) (BZY) proton-conducting electrolyte thin films by pulsed laser deposition on different single-crystalline substrates. Highly textured, epitaxially oriented BZY films were obtained on (100)-oriented MgO substrates, showing the largest proton conductivity ever reported for BZY samples, being 0.11 S cm(-1) at 500 °C. The excellent crystalline quality of BZY films allowed for the first time the experimental measurement of the large BZY bulk conductivity above 300 °C, expected in the absence of blocking grain boundaries. The measured proton conductivity is also significantly larger than the conductivity values of oxygen-ion conductors in the same temperature range, opening new potential for the development of miniaturized SOFCs for portable power supply.

Electrochemical NOx Sensors Based on Interfacing Nanosized LaFeO3 Perovskite-Type Oxide and Ionic Conductors

Electrochemical NOx sensors based on coupling different ionic conductors with perovskite-type LaFeO3 as auxiliary phase were prepared. The ionic conductors used were yttria-stabilized zirconia that is an oxygen ion conductor and sodium superionic conductor whose charge carriers are sodium ions. Thick films of LaFeO3 powders were deposited using a screen printing oil on one side of the solid electrolyte pellets, using powders having different grain sizes. The obtained sensors were wholly exposed to the same atmosphere. Electromotive force (emf), polarization curves, and amperometric measurements were performed in air and at different concentrations of NO2 in air, at selected temperatures. Quite stable emf values and fast response times were obtained. The NO2 response was much larger when powders with nanosized grains were used for the preparation of the electrodes. From polarization curves, all sensors showed a nonlinear behavior. The role played by the electrolyte/electrode interface was determined by electrochemical impedance spectroscopy measurements; exposure to NO2 did not affect the bulk but only the electrolyte/electrode interface.

Sensing Mechanism of Potentiometric Gas Sensors Based on Stabilized Zirconia with Oxide Electrodes Is It Always Mixed Potential

The non-Nernstian behavior of zirconia-based electrochemical NOx and CO sensors with various semiconducting oxides as sensing electrodes was extensively studied in the temperature range 450-700°C. Both pellets and tape-cast layers of yttriastabilized zirconia were used for the fabrication of the sensors. It was found that the gas-sensing mechanism of the sensors cannot be explained always by the mixed potential theory. When semiconducting oxides are used as sensing electrodes, the semiconductor sensing mechanism and the adsorption-desorption behavior of the different oxides dominate the sensor response if both metal and oxide are exposed to the same environment.

The effects of accelerated carbonation on CO2 uptake and metal release from incineration APC residues

This work presents the results of a study on accelerated carbonation of incinerator air pollution control residues, with a particular focus on the modifications in the leaching behaviour of the ash. Aqueous carbonation experiments were carried out using 100% CO(2) at different temperatures, pressures and liquid-to-solid ratios, in order to assess their influence on process kinetics, CO(2) uptake and the leaching behaviour of major and trace elements. The ash showed a particularly high reactivity towards CO(2), owing to the abundance of calcium hydroxides phases, with a maximum CO(2) uptake of approximately 250g/kg. The main effects of carbonation on trace metal leaching involved a significant decrease in mobility for Pb, Zn and Cu at high pH values, a slight change or mobilization for Cr and Sb, and no major effects on the release of As and soluble salts. Geochemical modelling of leachates indicated solubility control by different minerals after carbonation. In particular, in the stability pH range of carbonates, solubility control by a number of metal carbonates was clearly suggested by modelling results. These findings indicate that accelerated carbonation of incinerator ashes has the potential to convert trace contaminants into sparingly soluble carbonate forms, with an overall positive effect on their leaching behaviour.

Design and fabrication of a chemically-stable proton conductor bilayer electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs)

Bilayer electrolytes made of barium cerate covered with a thin film of barium zirconate deposited by pulsed laser deposition show promise for improving chemical stability without greatly affecting electrochemical performance in fuel cell operation.

Study of YSZ-based electrochemical sensors with WO3 electrodes in NO2 and CO environments

Electrochemical sensors based on yttria-stabilized zirconia (YSZ) with WO3 as a sensing electrode were fabricated using either Pt or Au electrodes. The sensors were studied in the temperature range 500-700°C in the presence of different concentrations (300-1000 ppm) of NO2 and CO in air. The response to NO2 was very stable with fast response time (within 20 s). The best sensitivity was observed at 600 and 650°C using Pt and Au electrodes, respectively. At all temperatures investigated a cross sensitivity to CO gas was also noticed. The response to CO was decreased using Au electrodes. The role played by WO3 as a sensing electrode was investigated.

Nano-structured perovskite oxide electrodes for planar electrochemical sensors using tape casted YSZ layers

Solid-state NO 2 sensors based on yttria stabilized zirconia (YSZ), an O 2- conductor, combined either with an n- (WO 3 ), or a p-type semiconducting oxide (LaFeO 3 ), or a mixed electronic and ionic conductor (La 0.8 Sr 0.2 FeO 3 ) were investigated. Platinum parallel finger electrodes were applied on the surface of tape-casted YSZ layers and attached with gold wires for current collection. Nanocrystalline perovskite powders were prepared using different chemical methods: LaFeO 3 by the thermal decomposition of the LaFe-hexacyanide complex, and La 0.8 Sr 0.2 FeO 3 by a sol-gel route. A sub-micrometric commercial WO 3 powder was used. The oxide powders were mixed with a screen-printing oil and deposited on one Pt finger electrode. The presence of the oxide powder makes one of the electrodes different from the other in terms of catalytic activity, specific surface area, gas adsorption and reaction kinetics. Both electrodes were wholly exposed to the same gas atmosphere, without using reference air. The sensors were investigated at fixed temperature (450-700 °C) by measuring the electromotive force (EMF) at different concentrations of NO 2 and CO in air in the range 20-1000 ppm. A fast and stable response was measured for all the tested sensors. An EMF of opposite sign was measured for p- and n-type semiconducting based sensors upon exposure to the same gas. After increasing the grain size of the nano-structured La 0.8 Sr 0.2 FeO 3 powder by a heat treatment at 900 °C for 4 h, the response to NO x became small, slow and unstable.

Effect of surface modification on NO2 sensing properties of SnO2 varistor-type sensors

NO2 sensing properties of SnO2-based varistor-type sensors have been investigated in the temperature range of 400-650 °C and in the NO2 concentration range of 15-30 ppm. Pure SnO2 exhibited a weak nonlinear I-V characteristic in air, but clear nonlinearity in NO2 at 450 °C. The breakdown voltage of SnO2 shifted to a high electric field upon exposure to NO2 and the magnitude of the shift was well correlated with NO2 concentration. Thus, SnO2 exhibited some sensitivity to NO2 as a varistor-type sensor. When SnO2 particles coated with a SiO2 thin film were used as a raw material for fabricating a varistor, the breakdown voltage in air was approximately the double that of pure SnO2 and the sensitivity to 15 ppm NO2 was enhanced slightly. However, the sensitivity to 30 ppm NO2 decreased. The Cr2O3-loading on SnO2 also led to an increase in the breakdown voltage in air, but the Cr2O3 addition was not effective for promoting the NO2 sensitivity under the present experimental conditions.

Preparation and Electrochemical Characterization of Perovskite/YSZ Ceramic Films

Perovskite-type La 0 . 8 Sr 0 . 2 Co 0 . 8 Fe 0 . 2 O 3 - δ powders were prepared using a complex polymeric precursor method. Thermal analysis was carried out on the perovskite precursor to investigate the oxide-phase formation. The structural phase of the powders was determined by X-ray diffraction. These results showed that the decomposition of the precursors occurs in a two-step reaction and temperatures higher that 1000°C are required for these decomposition reactions. For the electrochemical characterization, La 0 . 8 Sr 0 . 2 Co 0 . 8 Fe 0 . 2 O 3 - δ electrodes were deposited by a wet spray technique on dense yttria-stabilized zirconia (YSZ) layers. The morphology of the deposited perovskite thick films (∼50 μm) was investigated by field emission scanning electron microscopy and showed a porous microstructure. Electrochemical impedance spectroscopy (EIS) measurements were carried out under synthetic air flux at temperatures ranging from 200-600°C in the 10 mHz-10 MHz frequency range showing an interfacial electrical resistance related to the La 0 . 8 Sr 0 . 2 Co 0 . 8 Fe 0 . 2 O 3 - δ electrodes. EIS measurements were also performed in the same frequency range at different oxygen partial pressures (10 - 5 -1 atm) at 600°C. At this temperature and frequencies below 0.1 MHz, the electrical response to the applied signal of the electrode material is best fitted by two semicircles, which can be related to charge-transfer processes. The activation energy for the limiting step (adsorption/desorption) was found to be 1.6 eV.

Nanoscale mapping of oxygen vacancy kinetics in nanocrystalline Samarium doped ceria thin films

The position-dependent oxygen vacancy dynamics induced by a biased scanning probe microscopy tip in Samarium doped ceria thin films grown on MgO (100) substrates is investigated. The granularity of the samples gives rise to spatially dependent local electrochemical activity, as explored by electrochemical strain microscopy. The kinetics of the oxygen vacancy relaxation process is investigated separately for grain boundaries and grains. Higher oxygen vacancy concentration variation and slower diffusion are observed in the grain boundary regions as compared to the grains.